Method and system for controlling air temperature in an air conditioned zone

ABSTRACT

An air conditioning system for air circulating through an air conditioned zone is disclosed. A control system is provided which maintains a predetermined temperature in the zone by operating air heating and air chilling units of the system in either an onoff or a modulating mode. The system responds to the temperature of zone air and air flowing into the zone. Two air cooling units are disclosed, the first one of which can be modulated in its cooling effect and a second unit, having a constant capacity, is operated only if the cooling load exceeds the capacity of the modulatable unit. When the cooling load is less than the full capacity of both units, the first unit is modulated while the second unit operates at capacity. The air heating units are operated in stages. The sensed zone air temperature is primarily responsible for initiating cycles of the heating stages while the sensed duct air temperatures terminate operation of the heating stages. The heating stages have low thermal inertias so that they can be rapidly cycled.

United States Patent 1 Attridge, Jr.

[ METHOD AND SYSTEM FOR CONTROLLING AIR TEMPERATURE IN AN AIRCONDITIONED ZONE [75] Inventor: Russell G. Attridge, Jr., Columbus,

Ohio

[73] Assignee: Ranco Incorporated, Columbus,

Ohio

221 Filed: Nov. 9, 1970 [21] Appl. No.: 87,963

[451 July 17, 1973 Primary Examiner-Meyer Perlin AssistantExaminerRonald C. Capossela Attorney-Watts, Hoffmann, Fisher & Heinke[57] ABSTRACT An air conditioning system for air circulating through anair conditioned zone is disclosed. A control system is provided whichmaintains a predetermined temperature in the zone by operating airheating and air chilling units of the system in either an on-off or amodulating mode. The system responds to the temperature of zone air andair flowing into the zone.

Two air cooling units are disclosed, the first one of which can bemodulated in its cooling effect and a second unit, having a constantcapacity, is operated only if the cooling load exceeds the capacity ofthe modulatable unit. When the cooling load is less than the fullcapacity of both units, the first unit is modulated while the secondunit operates at capacity. The air heating units are operated in stages.The sensed zone air temperature is primarily responsible for initiatingcycles of the heating stages while the sensed duct air temperaturesterminate operation of the heating stages. The heating stages have lowthermal inertias so that they can be rapidly cycled.

12 Claims, 5 Drawing Figures METHOD AND SYSTEM FUR QQNTIRUILLKNG AERTEMPERATURE lN AN AIR CQNDHTIIGNED ZUNE CROSS REFERENCED US. PATENT U.S. Pat. No. 3,498,074 issued Mar. 3, l970 to Solomon S. Fineblum,entitled CONTROL SYSTEM FOR REFlllGERATING APPARATUS.

BACKGROUND OF THE lNVENTlON Field of the Invention This inventionrelates to comfort control systems and more particularly relates to amethod and system for controlling the operation of a comfort controlsystem to maintain a given temperature in an air conditioned zone.

The Prior Art The provision of a single large capacity refrigerationunit for cooling air introduced into the air conditioned zone has longbeen recognized as undesirable because cycling of such a unit,parh'cularly when the cooling load on the zone was not large, was toofrequent. Such systems were inefficient because relatively large amountsof electrical power consumed by frequent compressor cycling. Frequentcycling also subjected the equipment to deleterious stresses and reducedtheir life. Furthermore, the temperature of chilled air entering thezone was frequently undesirably low causing unduly large temperatureranges in the zone to be commonplace.

According, it was proposed that multistage refrigeration units beemployed to cool air conditioned zones. The cooling capacity of therefrigeration unit forming each stage was relatively small compared tothe total cooling capacity of the system. Comfort conditioning the airin the zone was then accomplished by, for example, operating one or morerefrigeration stages continuously and cycling additional stages, or bycycling a single stage, depending on the cooling load in the zone. Suchsystems did not obviate cycling refrigeration units but did reduce theinefficiencies incident to frequently cycling relatively largerefrigeration units.

Even when relatively small refrigeration units were employed in stages,the cooling capacity of such units increased substantially whenatmospheric air temperatures dropped below the atmospheric airtemperature at which the units were designed to operate. This was due toincreased heat rejection by condensers of the units. Standards fordesign of air conditioning systems required refrigeration units tooperate at their rated capacity at an outside atmospheric dry bulb airtemperature of 95 F". However, some manufacturers used an outside designtemperature of 105 F" to insure adequate cooling capacity at highambient temperatures. Thus when atmospheric air temperatures droppedbelow 95, the cooling capacities of refrigeration units increased withthe largest increases exhibited by units designed for rated capacity atl F. in some circumstances, the evaporator temperatures becamesufficiently cool to subcool the zone air. This caused wide temperatureexcursions in the air discharged into the zone and resulted in frequentcycling, excessive zone temperature fluctuations and uncomfortably coldduct discharge air streams.

The prior art recognized that if the capacity of the individualrefrigeration stages could be controlled, cycling frequency ofindividual units could be reduced. lit

was proposed to provide multiple unloader valves in the refrigerantcompressors of these units. Unloader valves operated to reduce thepressure of refrigerant at the compressor head, to decrease the unit'scooling capacity in proportion to the extent of refrigerant headpressure reduction.

When zone temperatures rose above a set point temperature, onerefrigeration stage was energized with a minimum refrigerant headpressure. As the zone continued to require more cooling, successiveunloader valves of the stage were operated so that the capacity of thefirst refrigeration stage was increased step-wise to full capacity.

if the zone required additional cooling, a second stage was energized,first at minimum cooling capacity and then step-wise to full capacitylike the first stage. The cooling load of the zone was balanced bycycling operation of one or more unloader valves rather than an entirerefrigeration stage.

There were a number of disadvantages to this last mentioned proposal.One disadvantage was that although small sensible cooling loads on thezone could be dealt with by operation of one stage at low capacity, thelatent heat load on the zone was not adequately controlled. That is, themoisture content of the zone air was not reduced sufficiently to productcomfort even though the zone air temperature was reduced. Anotherdisadvantage was that, even using unloader valves, the cooling systemcould not be matched with the cooling load on the zone because coolingcapacity was changed step-wise. Thirdly, because the capacity of eachrefrigeration unit was individually altered in the sequence outlinedabove, the control systems and refrigeration equipment required toperform these functions were relatively complex, and expensive topurchase and install.

Another shortcoming of some prior art control systems was theirinability to automatically govern both heating and cooling of the airconditioned zone. in some control systems, a mode switch had to bemanually operated from the zone. Separate thermostats for heating andcooling were also required in some systems.

SUMMARY OF THE INVENTHON The present invention provides a new method andsystem for automatically controlling air temperature in an airconditioned zone. The new control system is readily installable withoutexcessive labor costs. The new system is particularly suited forinclusion in combined heating and cooling air conditioning systemsthereby insuring optimum cooperation between the control system and theair conditioning unit with which it is installed.

Additionally, the new control system enables novel staging ofrefrigeration stages whereby the capacity of the refrigeration units ismodulatable so that the cooling'load can be substantially matched by thecooling capacity of the refrigeration stages. in the preferred system,modulation of the cooling capacity is accomplished by controlling thecooling capacity of a single refrigeration stage whether or not morethan one stage is operating; however modulation of the cooling capacityof additional stages is within the scope of this invention.

Modulation of refrigeration stages avoids short cycling of anyrefrigeration stage due to subcooling of air directed to a zone.Furthermore, modulation is accomplished by reducing the cooling capacityof the controlled unit from full capacity to a desired capacity. Thisassures that the latent heat load in the zone is maintained undercontrol.

In a preferred embodiment of the invention, an electrical temperaturesignal is amplified to produce an output signal for operating heating orcooling units as required to maintain the air in the zone at atemperature set point level. The input temperature signal is produced bya sensor unit in the zone and a sensor unit in a zone inlet air duct.

The temperature signal is a composite signal formed by combining thezone and duct sensor signals with a reference signal determined by thetemperature set point in the zone. The zone sensor signal has asubstantially larger authority" than the duct sensor signal; that is,the ratio of the zone sensor signal level produced as a result of asensed temperature change of, say 1 F, to the duct sensor signalproduced by the same sensed temperature change is relatively large. Thisauthority ratio may, on the average, be about :1.

The zone temperature sensor is principally responsible for initiatingoperation of the refrigeration stages. When the refrigeration stages areoperating, the duct sensor produces changes in the temperature signalwhich result in the cooling capacity of the refrigeration stages beingmodulated. Because of the relatively low authority of the duct sensorand the limited ability of refrigeration stages to cool duct air, theeffect of this sensor on the temperature signal is preferably not, ofitself, sufficiently great to cycle a refrigeration stage. The ductsensor does alter the temperature signal sufficiently to modulate thefirst refrigeration stage.

The new control system is preferably operated in connection withelectrical resistance heaters. When zoneheating is required by thecontrol system, the

heater unit cycling is controlled by the effect of the duct sensor onthe temperature signal. The electrical heater units, unlike therefrigeration stages, can be and are rapidly cycled when the zone isheated.

The heating units are constructed and arranged so that their cycling iscontrolled by the duct sensor so long as the zone temperature is belowthe set point temperature. The duct sensor thus functions to anticipatethe effect on zone air temperature by the heaters.

Because of the relatively low thermal inertia of resistance heatingunits, the zone temperature approaches the set point temperature as theheating units are cycled but does not tend to overshoot the set pointtemperature. When the set point temperature is reached, the heatingunits are maintained off by the zone sensor.

In a preferred embodiment, the system amplifier is associated with anelectrical power supply having a reference terminal and relativelypositive and negative terminals respectively. When the zone temperatureis higher than the set point temperature, the amplifier output signalvaries in one sense direction from the reference voltage level andoperates the refrigeration stages according to the signal level. Whenzone temperature is lower than the set point temperature, the amplifieroutput voltage varies from the reference voltage in the opposite sensedirection to control the zone heating equipment.

The refrigeration stages are turned on at two different amplifier outputvoltage levels and are maintained on as the amplifier output voltage isreduced toward the reference voltage. The refrigeration stages areturned off at different voltage levels. The voltage level at which thesecond stage is turned off is of lower magnitude than the level at whichthe first stage is turned on so that the first and second stages canoperate simultaneously. Overlapping the first stage turn on signal leveland the second stage turn ofi signal level enables operation of thesystem over a smaller range of signal levels than would be requiredwithout overlapping. The first and second stages can also be operatedsimultaneously at smaller signal levels.

The first stage is modulatable in response to amplifier output levelsranging between the level at which the first stage is turned on and thelevel at which the second stage turns off. This enables the first stagealone to be modulated from maximum capacity to a lesser level. The firststage is also modulated when the second stage is operating, enablingcooling capacities ranging downwardly from two stages.

Modulation is preferably accomplished by a vortex amplifier connected inthe compressor suction line to variably restrict the flow of refrigerantthrough the vortex amplifier to the compressor. The restriction producedby the vortex amplifier is governed by a pilot flow of condensedrefrigerant. The pilot flow rate is changed continuously or in astepwise fashion in response to changes in amplifier output levels andthereby modulates the cooling capacity of the first stage.

In a preferred embodiment of the invention, ventilation air is suppliedto the zone during operation of the first air conditioning stage so thatthe cooling effect produced by that stage is further modulated. Thequantity of ventilation air introduced into the zone is controlled inresponse to the temperature signal.

A principal object of the invention is the provision of a new andimproved air tempering system including a control system for operatingair heating and air cooling stages according to sensed air temperaturesand wherein the air cooling stages are modulated to minimize cycling ofrefrigeration units, and reduce inlet air temperature excursions.

Other objects and advantages of the invention will be apparent from thefollowing detailed description made with reference to the accompanyingdrawings which form a part of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammaticalillustration of an air conditioning zone, a system for conditioning theair in the zone and a comfort control system according to the inventionfor controlling the air conditioning system;

FIG. 2 is a diagrammatical illustration of the control system of FIG. 1;

FIG. 3 is a schematic illustration of a portion of the control system ofFIG. 2;

FIG. 4 is a schematic illustration of another portion of the controlsystem of FIG. 2; and,

FIG. 5 graphically depicts operation of the control system in relationto the zone air temperature.

DESCRIPTION OF A PREFERRED EMBODIMENT An air conditioning systemembodying the present invention is illustrated in FIG. 1. The airconditioning system 10 conditions air circulating through a zone 12 byway of ducting generally indicated at 14. The ducting 14 includes ablower unit 16 for circulating air through the ducting and zone. An airheating system 66 and an air cooling system 26 heat or cool the aircirculating through the ducting to the zone 112 as governed by a controlsystem 22.

The ducting 14 includes an air return section 241 through which airpasses from the zone into the ducting 14. An outdoor exhaust section 26communicates with the return section 24 and includes suitable dampers 28which govern the flow of air from the return section 24 to theatmosphere. Air flowing into the return section 24 may pass to a returnair branch 36 and toward the intake of the blower 16. An outdoor airinlet section 32 communicates with the return air branch 36 and the flowof outdoor air into the return branch 36 is controlled by dampers 34which are moved by a damper actuator 36. A discharge section 36 of theducting houses the blower 16, air cooling heat exchangers 42, 46 and airheating heat exchangers 41-6, 66. Air which is passed across the heatexchangers is discharged into the zone through grill opening 52.

The Heating System 18 The heating system 18 includes first and secondheating stages which are defined by the air heating heat exchangers 48,50. These heating stages preferably comprise electrically energizedbanks of resistance heated elements. Control switch units 66, 62 areconnected in the power circuits for the heat exchangers 46, 56,respectively, for controlling their energization. The control switchunits 60, 62 may be of any suitable construction and are not shown indetail. The switching units are constructed so that when either of theheat exchangers 48, 50 are energized, energization of individualresistance heated elements is staggered. Hence, excessive electricalpower is not consumed when either heat exchanger is initially energized.

Any number of heating stages can be provided in the system 18 dependingon the heating requirements of the zone.

The Cooling System 26 The air cooling system 26 includes first andsecond electrically powered refrigeration stages generally designated at66, 68. Each of these stages comprises a compressor-condenser-evaporatorrefrigeration unit including the air cooling heat exchangers 62, d4comprising the evaporators of the units 66, 66, respectively.

The first refrigeration stage 66 includes a refrigerant compressor 70having a high pressure side communicating with a condenser 71 through ahigh pressure line 72. Condensed refrigerant from the condenser 7H flowsthrough a condenser line 74 to an expansion valve 76. The refrigerantflowing through the expansion valve 76 vaporizes and passes through theevaporator 412 from which it is returned to the compressor 76 through asuction line 76.

The cooling capacity of the refrigeration stage 66 is modulated withinlimits by operation of a capacity control arrangement generallydesignated at 66. The capacity control includes a vortex amplifier 62connected in the suction line 78 between the evaporator 42 and thecompressor 70. The vortex amplifier is capable of variably restrictingthe refrigerant flow through the suction line 78 in accordance with theflow of refrigerant through a pilot flow line 64 The pilot flow throughthe line 66 is governed by a pilot control valve 66. The pilot valve 66is adapted to modulate the pilot flow of refrigerant introduced into thevortex amplifier 62 and thereby modulate the capacity of therefrigeration unit 66. When the pilot valve 66 prevents the pilot flowof refrigerant from entering the vortex amplifier, the refrigerationstage 66 operates at full capacity and as the pilot flow increasesthrough the pilot valve 66 the cooling capacity of the refrigerationstage 66 is reduced.

A capacity control arrangement for a refrigeration system similar to thecapacity control arrangement 66 is described in detail in the abovereferenced patent to Fineblum.

in the illustrated embodiment of the invention, the pilot flow isinfinitely variable by the pilot valve and infinitely varies the coolingcapacity of the stage 66 between limits. Alternately, the pilot valvecan vary the pilot flow in a stepwise fashion to modulate the coolingcapacity of the stage 66 in a stepwise manner.

The second refrigeration stage 68 is also thecompressor-condenser-evaporator type; however, in the illustratedembodiment, this stage is not provided with a capacity controlarrangement. The second stage 66 may be of any suitable construction andtherefore is shown schematically and not described in detail.

The Control System 22 The control system 22 governs operation of theheating or cooling stages and/or the damper actuator 36, depending uponthe duct discharge air temperature and upon zone air temperature. Apreferred control system is shown schematically in FIG. 2 and includes azone sensor unit 96, a duct air sensor unit 92 and a temperature setpoint adjusting element 94 combined with the zone sensor unit. Theseunits produce electrical temperature responsive signals which arecombined to provide a temperature responsive input signal to anamplifier 96. The amplifier 96 and the units 96, 92 are connected toterminals 97, 98, 99 of an accurately regulated electrical power supplywhich is illustrated as a direct current power supply.

The voltage level at the terminal 96 is a reference voltage and in theconvention used is sometimes referred to as Zero volts. The levels atthe terminals 97, 99, respectively, are referred to as plus voltage andminus voltage.

The amplifier 96 is capable of producing output signals which varybetween the plus and minus voltages depending on the level of thecombined temperature input signal to the amplifier. in the preferredembodiment, the amplifier output signals are direct current signals.

Generally speaking, when zone temperatures are higher than the desiredor set point level, for example F, the amplifier output voltage ispositive with respect to the reference voltage at the terminal 96. Whenzone temperatures fall below the set point temperature, the amplifieroutput signals tend to be negative with respect to the referencevoltage.

When the zone air temperature is above the set point temperature, theamplifier 96 provides a positive output signal which is transmitted tocooling control circuitry 1166 through an amplifier output lead Mill anda diode H62. The diode 1l62 passes only a positive output signal fromthe amplifier to the cooling control circuitry 1166.

The circuitry 666 includes a first refrigeration stage triggeringcircuit 1, a second refrigeration stage triggering circuit 166, a firststage capacity control circuit 1166 and a damper actuator controlcircuit 1H6.

The triggering circuits 104, 106 are preferably bistable voltagedetecting electronic switches which are responsible for activating theirrespective refrigeration stages at predetermined amplifier outputvoltage levels. These triggering circuits may be conventional andtherefore are not shown or described in detail. Control relays l 14, 116 are connected to the outputs of the trigger circuits 104, 106,respectively. These relays control the operation of the associatedrefrigeration stages 66, 68, respectively by energizing and deenergizingthe respective stages. This is accomplished, as is conventional, bycontrolling energization of the compressors of the refrigeration units.The control relays in the output circuits of each of the triggers 104,106 are connected across a suitable power supply via tenninals 104a,1064.

The capacity control arrangement 108 is shown schematically in FIG. 3.This circuit includes an electronic switch 120 which is connected inseries with a heat motor 122 across the full wave rectified unfilteredpower supply via a terminal 123. The heat motor 122 includes anelectrical heater element 126 and a bimetal actuator 128 for operatingthe refrigerant pilot flow valve 86. As the heat transferred from theheater 126 to the bimetal actuator 128 is increased, the pilot valve isactuated to throttle the pilot flow to the vortex amplifier therebycausing the cooling capacity of the refrigeration stage 66 to increasetowards full capacity. When heat transfer between the heater 126 andactuator 128 is at a relatively low level, the actuator 128 opens thevalve 86 to reduce the capacity of the refrigeration stage 66.

The electronic switch 120 governs energization of the heater 126primarily in response to the command signal from the amplifier 96. Asshown in FIG. 3, the switch 120 includes Silicon Controlled Rectifiers[SCRs] 129, 130 which are connected in parallel with each other and inseries with the heater element 126. The SCR 130 is connected around theSCR 129 through a voltage dropping resistor 131 so that when the SCR 130conducts, the power supply voltage falls across the heater 126 and theresistor 131. This prevents operative energization of the heater whenthe SCR 130 is conductive. When the SCR 129 conducts the resistor 131and the SCR 130 are shunted and the heater is operatively energized,i.e., nearly the entire power supply voltage falls across it.

The conductive stages of the SCRs 129, 130 are controlled by an errorcircuit 132 which includes a resistance bridge and a differentialamplifier. The differential amplifier includes transistors 134, 136having emitters 138, 140, respectively, connected to the power supplyterminal 123 through an emitter resistor 142, the voltage droppingresistor 131, a diode 146 and the heater element 126. The collectorelectrodes 148, 150 of the transistors are each connected to groundthrough output resistors 151, 152, respectively.

The base 153 of the transistor 136 is connected to the wiper 154 of apotentiometer 156 connected in one arm of the error bridge. Thepotentiometer 156 is connected in series with bridge resistors 158, 160between the voltage dropping resistor 131 and ground. The resistors 158,160 and the potentiometer 154 provide a biasing voltage level at thebase 153 of the transistor 136 and determine the voltage level at theemitter 140 which is required to turn on the transistor 136. Once thewiper 154 of the potentiometer 156 is set at a particular location thetransistor 136 will be rendered conductive at substantially the samepoint during each pulsation of the power supply.

The base voltage signal of the transistor 134 is thus controlled by theoutput of the amplifier 96, or command signal, and the state of theerror bridge. The base 162 of the transistor 134 is connected to theoutput of the amplifier 96 via a junction 164 and conductor 166. Thejunction 164 is located in a bridge arm defined by a temperatureresponsive resistor 168 and a resistor 170 connected between theresistor 131 and ground.

The transistor 134 turns on during each power supply pulsation at a timedetermined by the voltage level at the junction 164. The voltage levelat the junction 164 is influenced by both the bridge arm resistance andthe level of the amplifier output signal on the conductor 166.

If the voltage level at the junction 164 is less than the voltage levelat the base 153 of the transistor 136 during any given power supplypulsation, the transistor 134 will be rendered conductive before thetransistor 136 conducts and will conduct more heavily than thetransistor 136 at any given instant during a power supply pulsation. Onthe other hand when the voltage level at the junction 164 tends to begreater than the voltage level at the base 153, the transistor 136 isrendered conductive earlier in the power supply pulsation than thetransistor 134 and will conduct more heavily at any given time in thepower supply pulsation.

The collector electrode 148 of the transistor 134 is connected to thegate electrode of the SCR 130. The collector 150 of the transistor 136is connected to the gate electrode of the SCR 129. When conductionthrough the transistor 136 is sufficiently great that the voltage acrossthe output resistor 152 reaches the triggering level of the SCR 129, theSCR 129 is rendered conductive to establish an energizing circuit fromthe power supply terminal 123 through the heater 126 and to ground.

Once the SCR 129 is rendered conductive, the diode 146, resistor 131,and the error circuit 132 are shunted by the SCR 129. This preventsturning on of the transistor 134 during any power supply pulsation inwhich the SCR 129 becomes conductive. When the SCR 129 conducts, theheater element 126 is energized to heat the actuator 128 throughoutpower supply pulsations during which the SCR 129 conducts.

During power supply pulsations when the transistor 134 is renderedconductive before the transistor 136, the SCR 130 is rendered conductiveto establish a circuit from the power supply terminal 123 through theheater 126, diode 146, resistor 131, and to ground. When the SCR 130 isconductive, the small forward voltage drop across it prevents thetransistor 136 from triggering the SCR 129 during that power supplypulsation.

When the SCR 130 conducts, the voltage across the heater 126 isrelatively small because of the size of the voltage dropping resistor131. For this reason the heater 126 is substantially unheated throughoutpower supply pulsations during which the SCR 131 conducts.

The switch renders either of the SCRs conductive substantiallythroughout each power supply pulsation and is therefore termed a zero"switch, since it conducts at nearly a zero power supply phase angle.When the amplifier output voltage, or command signal, is at or above apredetermined level the SCR 129 is conductive substantially continuouslyso that the cooling capacity of the refrigeration unit 66 is maximum. Asthe command signal is reduced in magnitude the SCR 129 is conductivethroughout progressively fewer power supply pulsations and the pilotvalve 86 is progressively opened thereby gradually reducing the coolingcapacity of the unit 66 as the command signal is reduced. The coolingcapacity can be reduced to nearly any desired level according to thedesign of the parts of the capacity control arrangement 108. Suffice itto say that in the preferred embodiment modulation of the first stagecooling capacity reduces the cooling capacity by as much as 50 percentof full capacity as the command signal varies between predeterminedlimits.

The resistor 168 has a positive temperature coefficient of resistanceand provides a temperature feedback arrangement for the switch 120 sothat the heater 126 is not excessively heated or permitted to become toocool. The thermistor 168 is located in heat exchange relationship withthe heater 126 so that when the heater is heated the thermistor 168 islikewise heated. Heating of the resistor 168 tends to decrease thevoltage level'at the base 162 of the transistor 134 and thus tends toturn on the transistor 134 relatively early in each power supplypulsation. This tends to reduce the heating rate of the heater 126.

On the other hand, when the heater 126 is not energized, the resistor168 cools resulting in a reduction in its resistance. The reducedresistance tends to retard conduction of the transistor 134, thusincreasing the heat input to the heater.

The actuator control circuit 1 is operated from the amplifieroutputsignal and controls operation of the damper actuator 36. Hence theactuator control circuitry modulates the ventilation air entering thezone.

. erence voltage, heating control circuitry 175 is energized to controlheating of the zone. The heating control circuitry is connected to theamplifier output lead 101 through a filter diode 176 which is forwardlybiased when the amplifier output voltage is negative. The filter diodeconnected between the cooling control circuitry and the amplifier 96 isnonconductive during this period so that the cooling control circuitryis deenergized.

The heating control circuitry 175 includes electronic triggeringcircuits 177, 178 which operate the heating stage control switches 60,62, respectively, by way of control relays. The triggering circuits 177,178 are preferably bistable voltage detecting switches which may besimilar to those referred to above in connection with the coolingcontrol circuitry 100. These triggering circuits operate theirrespective heating stage control switches at differing negativeamplifier output voltage levels to provide staged heating.

As shown in FIG. 4, the amplifier 96 is a differential amplifier havinga temperature signal input formed by a temperature input signal lead orconductor 181. The lead 181 connects the amplifier 96 to the duct sensorcircuitry 182 and zone sensor circuitry 183 through a junction 184.

The duct sensor circuitry provides a duct temperature signal to theamplifier input lead 181 which is a function of duct air temperature.The duct sensor circuitry 192 includes a thermally responsive resistancestring formed by a duct sensor thermistor 166 located in the duct sensorunit 92, a resistor 187 connected in series with the thermistor 186 anda resistor 188 which is connected parallel to the thermistor 166. Anoutput junction 169 of the resistance string is connected between theresistor 187 and the thermistor 166. The output junction 189 providesthe duct temperature dependent signal to the amplifier 96.

The magnitude of the duct temperature signal is determined by thesetting of a potentiometer 1911. The output junction 189 of theresistance string is connected to the amplifier input lead 161 through aresistor 192 and the potentiometer 196.

The zone temperature circuitry includes a thermally responsiveresistance string connected to the input 161 of the amplifier 96. Thezone temperature sensor string is connected across the terminals 97, 99and includes a thermistor 195, potentiometer 196 and resistor 197 whichare connected in series across the power supply terminals. A resistor198 is connected in parallel with the thermistor 195. The zonetemperature resistance string is connected to the amplifier input lead161 through an output junction 26!) and a lead 2111 connecting theoutput junction 200 to the junction 1.

The potentiometer 196 includes a wiper 292 which is movable to providefor a set point adjustment of the zone temperature. The wiper 202 isconnected to a suitable knob 203 which is moved to adjust the signallevel at the junction 2116 and thereby adjust the set point temperaturein the zone.

The amplifier 96 further includes a reference voltage input lead 219which is connected to an output junction 211 of a voltage dividercircuit. The voltage divider circuit is formed by series connected fixedresistors 214, 216 which are connected in series across the power supplyterminals 97, 99.

The reference voltage at the junction 21 1 is provided to the referenceinput of the difierential amplifier 96 so that the output signalproduced by the amplifier 96 has a level which is proportional to thedifference between the voltage levels at the reference input lead 216and the temperature signal input lead 181. The reference and inputsignals are calibrated during manufacturing. As shown in FIG. 4, avoltage divider 217 is connected across the power supply terminals 97,99. This voltage divider consists of a resistor 216, a potentiometer2241 and a resistor 222. The wiper 224 of the potentiometer 226 isconnected to the input reference lead 219 and this wiper is positionedduring manufacture to calibrate the reference signal.

Suitable gain adjusting circuitry 239 provides negative feedback for theamplifier 96. The gain adjusting circuitry 239 includes a potentiometerenabling adjustment of the amplifier gain. The amplifier gain ispreferably adjusted only during installation of the system.

FIG. 1 schematically shows the zone and duct sensors associated withcontrol system 22. The zone sensor unit and set point adjusting element94 are disposed in a housing located in the zone 12. The duct sensorunit 92 is a separate housing in the air discharge section of theducting 14.

It is apparent, particularly from inspection of the FIG. 1, thatwhenever one or both refrigeration stages 66, 68 operates, the ductsensor unit 92 is subjected to substantially greater air temperaturevariations than is the zone sensor unit 90. This is because the airpassing through the air cooling heat exchanger impinges on the ductsensor. The zone sensor is remote from the air cooling heat exchangersand is, if properly positioned in the zone, not directly impinged on byair discharged into the zone. To avoid rapid cycling of one or morerefrigeration stages, the temperature signal produced by the duct sensorhas a smaller authority than the signal produced by the zone sensor inresponse to an identical sensed air temperature change. That is, theeffect of temperature changes on the amplifier input signal due totemperature change sensed by the duct sensor is small relative to theeffect on the amplifier input signal caused by the same temperaturechange detected by the zone sensor 90.

The input signal to the amplifier 96 can vary according to the zonesensor unit signal as well as according to the duct sensor signal. Thesesignals are not necessarily dependent on one another. Hence, the outputsignal from the amplifier 96 can be influenced primarily by eithersignal or both, depending on the operating condition of the system 22.The low authority of the duct sensor enables the refrigeration stages tobe operated to cool the air in the zone without the stages being cycledin response to reduced duct air temperatures which do not reflect thezone air temperature.

The authority of the duct sensor signal in altering the composite inputsignal to the amplifier 96 is preferably relatively small as compared tothe authority of the zone sensor signal. In a preferred system,experimentally operated, the change in amplifier input signal producedby sensing l of zone air temperature change was 20 times the change inthe input signal produced by sensing a 1 duct air temperature change.Expressed as a ratio, the authority of the zone sensor to the ductsensor is preferably 20:1.

The authority ratio can be changed by altering the impedance of thepotentiometer 194 to change the strength of the duct sensor signalrelative to the zone sensor signal. This adjustment is preferably onlymade in connection with the installation or servicing of a system in agiven building.

Operation of the System 22 for Cooling the Zone The operation of apreferred system is graphically illustrated in FIG. 5. FIG. 5 shows agraph having an ordinate corresponding to the output voltage of theamplifier 96 expressed as percentages of the voltage difference betweenthe amplifier output voltage and the neutral power supply terminal 98,or zero volts. Thus the amplifier output voltage is shown as rangingbetween plus and minus 100 percent voltage. The abscissa corresponds tosensed zone temperatures differing from a set point temperature at theorigin. The set point temperature may be, for example 75.

An amplifier output signal is indicated by the line 300 whichillustrates the amplifier output voltages as a function of zonetemperature only. From FIG. 5 it will be seen that when the sensed zoneair temperature increases l.5 F above the set point temperature theamplifier output voltage increases linearly to about 100 percentvoltage. Likewise when the zone air temperature drops 1.5" below the setpoint temperature the amplifier output voltage is reduced to about l00percent.

The duct sensor signal component of the amplifier input signal issubstantially independent of the zone sensor signal (within thelimitations of the system equipment). Accordingly, at any particularlocation on the curve 300, a signal from the duct sensor unit indicatinga change in temperature of the duct air will alter the magnitude of theamplifier output voltage according to the sensed direction of thedetected air tempera- .ture change. The authority ratio of the zonesensor signal to the duct sensor signal is 20:1, when the duct sensordetects a change in duct air temperature reduction of 3 the amplifieroutput voltage is reduced by about 10 percent voltage.

Referring now to FIG. 5, assume that the zone temperature set point isadjusted to F. Assume further that the sensed zone air temperature is ator close to 75 F and that the cooling load in the zone is increasing andtherefore the zone temperature is increasing.

When the zone air temperature has increased to about 756 F the amplifieroutput signal is at around 40 percent and the damper actuator 36 isoperated so that the blower 16 circulates outside air to the zonethrough the damper 34 tending to stabilize the zone air temperature. Ifthe zone air temperature is reduced to a level just above the set pointtemperature (about 10 percent of the amplifier output voltage) thedampers reclose.

In the preferred embodiment, the damper actuator operates the dampers inproportion to the magnitude of the amplifier output voltage. Hence, thedampers are fully opened when the ventilation cycle is initiated andprogressively close toward a minimum ventilation position as theamplifier output voltage is reduced toward the set point level. In thepreferred embodiment, the dampers are completely closed when the blowerunit 16 is off and whenever the blower unit operates the dampers aremoved to a minimum ventilation position. The dampers are operatedbetween the minimum ventilation position and the fully open positionduring the ventilation cycle.

If the cooling load on the zone continues to increase during theventilation cycle, the sensed temperature in the zone increases stillfurther above the set point temperature unu'l the zone sensor detects anincrease in zone air temperature of about 1.2F above the set pointlevel. This causes the output voltage of the amplifier 96 to increase toabout percent. This voltage level is sufficient to operate thetriggering circuit 104 of the first refrigeration stage 66 whereupon thefirst stage is energized and operated at full capacity resulting inchilled air being directed through the duct and into the zone.

The chilled air flowing into the zone impinges on the duct sensor unit92 and causes a reduction of the duct sensor output signal. This in turnreduces the amplifier output voltage at a rate of 10 percent for every 3of air temperature reduction sensed by the duct sensor.

The amplifier output voltage therefore drops along the broken line curve302 in response to the reduction in the output signal from the ductsensor. The curve 302 is shown offset from the amplifier output voltagelevel at which the stage 66 is operated since the first refrigerationstage 66, even operating at full capacity, does not immediately stop theincreasing zone temperatures.

When the duct sensor output signal has reduced the output voltage of theamplifier 96 to between 60 and 70 percent of its maximum positive outputvoltage, the capacity control circuit 108 begins reducing the coolingcapacity of the stage 66. This reduction in cooling capacity tends tostabilize the amplifier output voltage by stabilizing the duct airtemperature.

If the duct sensor temperature is reduced sufficiently to drop theamplifier output voltage below the lower level of the capacity controlmodulation range, the stage 66 is operated at minimum capacity while thezone continues to be cooled.

If the cooling load on the zone is balanced by the first stage 66operating in the capacity modulating range, or below that range atminimum capacity, the unit 66 continues to operate indefinitely. If thefirst unit 66, operating at minimum capacity, reduces the zone airtemperature to about 75.5 F the unit 66 cycles. The compressor 70 ispreferably turned off when the combined signal levels of the duct sensorand the zone sensor produce an amplifier output voltage of about 30percent of maximum voltage.

If the first stage 66, operating at maximum capacity, is not capable ofcarrying the cooling load on the zone, the zone air temperaturecontinues to increase. The zone temperature increases until the combinedduct sensor and zone sensor signals produce an amplifier output voltageof around 100 percent. At this juncture the second stage 68 is operated.

.Sirnultaneous operation of the first and second stages 66, 68 causes amarked reduction in the duct air temperature. Accordingly, the ductsensor output signal is substantially reduced and the amplifier outputvoltage falls as is shown by the broken line curve 304 in FIG. 5.

The reduction in the amplifier output voltage may be sufficiently largethat the amplifier output voltage level is reduced into the capacitycontrol modulation range. When this occurs, the capacity of the firstrefrigeration stage is again modulated to stabilize the amplifier outputvoltage level in the modulation range. In the illustrated embodiment,the second stage operates at full capacity at all times.

If the cooling load in the zone is balanced when the first unit coolingcapacity is modulated with the second unit operating at full capacity,both stages are continuously operated and the amplifier output signal isstabilized in the modulation range. This prevents the air dischargedinto the zone from being undesirably subcooled while avoiding cycling ofthe second stage.

If the zone temperature is reduced by simultaneous operation of bothstages (with the first stage operating at minimum capacity) the secondstage is turned off when the amplifier output voltage is reduced belowabout 50 percent of maximum voltage. Thereafter the zone continues to becooled by the first stage.

Because the first stage cooling capacity is modulated while the secondstage is operating at full capacity, the cooling capacity of thecombined stages is thereby "modulated. This minimizes cycling of thesecond stage.

Furthermore, because of the relatively low authority of the duct sensor,cycling the refrigeration stages occurs in response to sensed 'zone airtemperature changes relative to the set point temperature and not as aresult of changes in sensedduct air temperatures. The cooling capacitymodulation, along with the relatively small authority of the duct sensorfurther reduce cycling of the refrigeration stages.

-From the preceding description it should be appreciated that byoverlapping the signal ranges in which the individual cooling stages areoperated and by providing cooling capacity modulation throughout asignal range within the overlapped cooling stage signal ranges, thecooling capacity afforded by multiple refrigeration units can bemodulated by modulating only the first stage refrigeration unit. Thisminimizes cycling of stages, reduces the complexity of the controlsystem required to operate the stages and enables the use of relativelyunsophisticated refrigeration units.

While only two refrigeration units are provided in the illustratedcomfort control system, it should be apparent that additionalrefrigeration stages can be provided. Operation of the System 33 forHeating the Zone When the zone temperature drops below the set pointlevel, the amplifier output signal becomes negative with respect to thevoltage of the power supply terminal 98 and increases in magnitudenegatively as a function of the reduction of sensed zone air temperaturebelow the set point temperature. The air heating stages are cycled as afunction of the amplifier output voltage levels as shown in FIG. 5. Theheating stages each have a relatively small thermal inertia and thusheat rapidly when energized and cool quickly when they are deenergized.

The control system 22 functions to cycle the heating stages bydeenergizing the heater elements in response to the duct sensor signalwhile reenergizing them principally in response to the zone temperaturesignal. The amplifier output levels at which the heating stages areindividually energized and deenergized are relatively closely spaced.The first stage is energized at a negative output voltage level of 40percent and is deenergized at minus 10 percent. The second heater stageis energized at minus percent and deenergized at minus 30 percent.

When the sensed zone air temperature is reduced about 0.5 F below theset point temperature, the amplifier output voltage level increasesnegatively to about 40 percent of its maximum and the first heatingstage is energized. The heating unit rapidly produces a flow of hot airthrough the duct discharge and into the zone. The temperature of thedischarge air is sensed by the duct sensor unit. The duct airtemperature is increased sufficiently by the heating stage that thetemperature signal level produced by the duct sensor alone causes theheating stage to be deenergized.

The deenergized heating unit rapidly cools resulting in the duct sensorsignal being quickly restored to a level corresponding to the zone airtemperature. When this occurs the zone sensor signal again causes theamplifier to reenergize the first heating stage. The first stage heateris thus cycled at a frequency sufficient to maintain the zonetemperature within about lF of the set point temperature.

If the heating load on the zone increases, the zone temperature isreduced further. When the amplifier output voltage level is at aboutminus 60 percent of maximum, the second heating stage is energized sothat both the first and second heating stages are simultaneouslyenergized.

When both stages are energized, the discharge air temperature sensed bythe duct sensor causes the second heating stage to be deenergizedrelatively quickly. The first stage may be maintained energized due tothe effect of the sensed zone air temperature on the combinedtemperature input signal to the amplifier. The second heating stage isthus cycled relatively frequently as a result of operation of the ductsensor.

Due to the low thermal inertia of the heating stages and cooperationbetween the duct sensor unit and the zone sensor unit in cycling theheating stages the zone temperature is stabilized without anysubstantial overshooting of the zone temperature above the set pointlevel.

While a single embodiment of the invention has been illustrated anddescribed in considerable detail, the present invention is not to beconsidered to be limited to the precise construction shown. Otheradaptations, modifications and uses of the invention may occur to thoseskilled in the art and it is intended to cover hereby all suchadaptations, modifications and uses which fall within the scope of theappended claims.

What is claimed is:

l. A method of controlling air temperature in an air conditioned zonecomprising:

a. providing at least first and second refrigeration units;

b. directing air across air cooling heat exchangers of said units andinto said zone through a zone air inlet;

c. producing a zone air temperature signal which varies in magnituderelative to a reference level in accordance with fluctuations in sensedair temperature in said zone;

d. producing an air temperature signal varying in magnitude relative tosaid reference level in accordance with fluctuations in sensedtemperature of air supplied to said zone;

. combining said zone and supply air temperature signals to produce acomposite cooling temperature signal varying in magnitude relative tosaid reference level;

f. initiating operation of said first refrigeration unit in response toproduction of a first composite cooling temperature signal having apredetermined magnitude relative to said reference level and terminatingoperation of said first unit in response to production of a secondcomposite temperature signal having a second predetermined magnitudeless than the magnitude of said first signal;

g. initiating operation of said second refrigeration unit in response toa third composite temperature signal level having a greater magnitudethan said first signal level and terminating operation of said secondunit at a fourth composite temperature signal level having a greatermagnitude than said second composite signal level and a lesser magnitudethan said first composite signal level; and,

h. modulating the cooling capacity of said first unit between fullcapacity and a lesser capacity in accordance with composite temperaturesignals ranging between fifth and sixth levels, said range of modulatingcomposite signals between said fifth and sixth levels being of lessermagnitude than said first signal level and greater magnitude than saidfourth signal level whereby the capacity of said first refrigerationunit is fully modulatable when said first unit is operating and whensaid first and second units are operating.

2. A method as claimed in claim 1 wherein modulating said coolingcapacity includes variably restricting refrigerant flow in said firstrefrigeration unit.

3. A method as claimed in claim 1 and further including providingventilating air to said zone in proportion to changes in compositetemperature signals between said reference level and a seventh compositesignal level less than said second signal level.

4. A method as claimed in claim 1 wherein said first refrigeration unitis initially operated at full capacity and is modulated from fullcapacity as said composite signal level is reduced from said fifthtoward said sixth level.

5. A method as claimed in claim 1 wherein said zone supply airtemperature signal and said zone temperature signal are algebraicallycombined to produce said composite signal, and wherein a predeterminedchange in zone supply air temperature produces a small change inmagnitude of said supply air temperature signal as compared to thechange in magnitude of said zone temperature signal produced by the zoneair temperature undergoing said predetermined change.

6. A method as claimed in claim 5 wherein operation of saidrefrigeration units is initiated primarily in response to zone airtemperature signal levels and the cooling capacity of said units ismodulated primarily according to zone supply air temperature signallevels.

7. A method as claimed in claim 5 wherein the authority ratio of saidzone air temperature signal to said supply air temperature signal isabout 20:1 when said zone air temperature and a zone set pointtemperature are the same.

8. In a system for cooling air in a zone having at least first andsecond refrigeration units and capacity control means for variablycontrolling the cooling capacity of at least said first refrigerationunit:

a. first air temperature sensing means exposed to air in said zone;

b. second air temperature sensing means exposed to air supplied to saidzone;

c. first and second signal responsive means for operating said first andsecond signal refrigeration units, respectively;

d. a third signal responsive means for operating said capacity controlmeans;

signal generating means having an input connected to said first andsecond air temperature sensing means and an output connected to saidfirst, second and third signal responsive means;

f. said first and second air temperature sensing means producing airtemperature responsive signals and said signal generating meansresponding to air temperature signals from said first and second airtemperature sensing means to produce a temperature responsive outputsignal applied to said signal responsive means;

. said first signal responsive means effective to initiate operation ofsaid first refrigeration unit at a first output signal level and toterminate operation of said first unit at a second output signal levelof lesser magnitude than said first signal level;

h. said third signal responsive means operating said capacity controlmeans to modulate the capacity of said first refrigeration unit betweenthird and fourth output signal levels of lesser magnitude than saidfirst output signal level and greater magnitude than said second outputsignal level; and,

i. said second signal responsive means effective to initiate operationof said second unit at a fifth output signal level having a greatermagnitude than said first output signal level and terminating operationof said second unit at a sixth output signal level of lesser magnitudethan said third and fourth output signal levels;

j. said third signal responsive means modulating the capacity of saidfirst unit between said third and fourth output signal levels duringoperation of said first unit alone and when both of said first andsecond units are operating.

9. A system as claimed in claim 8 further including summing circuitryconnecting said first and second air temperature sensing means to saidsignal generating means, said summing circuitry applying the algebraicsum of temperature signals from said first and second air temperaturesensing means to said signal generating means, said second airtemperature sensing means having a relatively small authority ascompared to said first air temperature sensing means. a

10. A system as claimed in claim 8 wherein said signal generating meansproduces a variable voltage output signal, said first and second signalresponsive means comprise voltage responsive bistable electronicswitches for energizing and deenergizing respective electrically poweredrefrigeration units, and said capacity control means comprises arefrigerant flow controlling unit associated with said firstrefrigeration unit for variably controlling the flow of refrigerant insaid first refrigeration unit to thereby control the cooling capacity ofsaid unit in response to operation of said third signal responsivemeans.

11. A system as claimed in claim 10 and further including zone set pointadjusting circuitry for establishing an adjustable zone set pointtemperature, said zone set point adjusting circuitry comprising anadjustable impedance element for shifting the level of said airtemperature responsive signals relative to a reference voltage tothereby change the zone set point temperature, and wherein the levels ofsaid first through sixth output signals vary from said reference voltagelevel in one sense direction.

12. A method of controlling air temperature in an air conditioned zonecomprising:

a. providing first and second refrigeration units;

b. directing air across air cooling heat exchangers of said units andinto said zone;

c. producing an electric air temperature responsive signal which variesin magnitude relative to a reference level in accordance withfluctuations in sensed air temperature, said air temperature responsivesignal increasing in magnitude relative to the reference level inresponse to increases in sensed air temperature and decreasing inmagnitude relative to the reference level in response to decreases insensed air temperature;

d. initiating operation of said first refrigeration unit in response toproduction of a first air temperature signal having a first magnituderelative to said reference level and terminating operation of said firstunit in response to production of a second air temperature signal havinga second magnitude less than the magnitude of said first signal;

e. initiating operation of said second refrigeration unit in response toa third air temperature signal level having a greater magnitude thansaid first air temperature signal level and terminating operation ofsaid second unit at a fourth air temperature signal level having agreater magnitude than said second air temperature signal level and alesser magnitude than said first air temperature signal level; and,

f. modulating the cooling capacity of said first unit between fullcapacity and a lesser capacity in accordance with air temperaturesignals ranging between fifth and sixth levels, said range of modulatingsignals between said fifth and sixth levels being of lesser magnitudethan said first air temperature signal level and greater magnitude thansaid fourth air temperature signal level whereby the capacity of saidfirst refrigeration unit is fully modulatable when said first unit isoperating and when said first and second units are operating.

1. A method of controlling air temperature in an air conditioned zonecomprising: a. providing at least first and second refrigeration units;b. directing air across air cooling heat exchangers of said units andinto said zone through a zone air inlet; c. producing a zone airtemperature signal which varies in magnitude relative to a referencelevel in accordance with fluctuations in sensed air temperature in saidzone; d. producing an air temperature signal varying in magnituderelative to said reference level in accordance with fluctuations insensed temperature of air supplied to said zone; e. combining said zoneand supply air temperature signals to produce a composite coolingtemperature signal varying in magnitude relative to said referencelevel; f. initiating operation of said first refrigeration unit inresponse to production of a first composite cooling temperature signalhaving a predetermined magnitude relative to said reference level andterminating operation of said first unit in response to production of asecond composite temperature signal having a second predeterminedmagnitude less than the magnitude of said first signal; g. initiatingoperation of said second refrigeration unit in response to a thirdcomposite temperature signal level having a greateR magnitude than saidfirst signal level and terminating operation of said second unit at afourth composite temperature signal level having a greater magnitudethan said second composite signal level and a lesser magnitude than saidfirst composite signal level; and, h. modulating the cooling capacity ofsaid first unit between full capacity and a lesser capacity inaccordance with composite temperature signals ranging between fifth andsixth levels, said range of modulating composite signals between saidfifth and sixth levels being of lesser magnitude than said first signallevel and greater magnitude than said fourth signal level whereby thecapacity of said first refrigeration unit is fully modulatable when saidfirst unit is operating and when said first and second units areoperating.
 2. A method as claimed in claim 1 wherein modulating saidcooling capacity includes variably restricting refrigerant flow in saidfirst refrigeration unit.
 3. A method as claimed in claim 1 and furtherincluding providing ventilating air to said zone in proportion tochanges in composite temperature signals between said reference leveland a seventh composite signal level less than said second signal level.4. A method as claimed in claim 1 wherein said first refrigeration unitis initially operated at full capacity and is modulated from fullcapacity as said composite signal level is reduced from said fifthtoward said sixth level.
 5. A method as claimed in claim 1 wherein saidzone supply air temperature signal and said zone temperature signal arealgebraically combined to produce said composite signal, and wherein apredetermined change in zone supply air temperature produces a smallchange in magnitude of said supply air temperature signal as compared tothe change in magnitude of said zone temperature signal produced by thezone air temperature undergoing said predetermined change.
 6. A methodas claimed in claim 5 wherein operation of said refrigeration units isinitiated primarily in response to zone air temperature signal levelsand the cooling capacity of said units is modulated primarily accordingto zone supply air temperature signal levels.
 7. A method as claimed inclaim 5 wherein the authority ratio of said zone air temperature signalto said supply air temperature signal is about 20:1 when said zone airtemperature and a zone set point temperature are the same.
 8. In asystem for cooling air in a zone having at least first and secondrefrigeration units and capacity control means for variably controllingthe cooling capacity of at least said first refrigeration unit: a. firstair temperature sensing means exposed to air in said zone; b. second airtemperature sensing means exposed to air supplied to said zone; c. firstand second signal responsive means for operating said first and secondsignal refrigeration units, respectively; d. a third signal responsivemeans for operating said capacity control means; e. signal generatingmeans having an input connected to said first and second air temperaturesensing means and an output connected to said first, second and thirdsignal responsive means; f. said first and second air temperaturesensing means producing air temperature responsive signals and saidsignal generating means responding to air temperature signals from saidfirst and second air temperature sensing means to produce a temperatureresponsive output signal applied to said signal responsive means; g.said first signal responsive means effective to initiate operation ofsaid first refrigeration unit at a first output signal level and toterminate operation of said first unit at a second output signal levelof lesser magnitude than said first signal level; h. said third signalresponsive means operating said capacity control means to modulate thecapacity of said first refrigeration unit between third and fourthoutput signal levels of lesser magnitude than said first output signallevel and greatEr magnitude than said second output signal level; and,i. said second signal responsive means effective to initiate operationof said second unit at a fifth output signal level having a greatermagnitude than said first output signal level and terminating operationof said second unit at a sixth output signal level of lesser magnitudethan said third and fourth output signal levels; j. said third signalresponsive means modulating the capacity of said first unit between saidthird and fourth output signal levels during operation of said firstunit alone and when both of said first and second units are operating.9. A system as claimed in claim 8 further including summing circuitryconnecting said first and second air temperature sensing means to saidsignal generating means, said summing circuitry applying the algebraicsum of temperature signals from said first and second air temperaturesensing means to said signal generating means, said second airtemperature sensing means having a relatively small authority ascompared to said first air temperature sensing means.
 10. A system asclaimed in claim 8 wherein said signal generating means produces avariable voltage output signal, said first and second signal responsivemeans comprise voltage responsive bistable electronic switches forenergizing and deenergizing respective electrically poweredrefrigeration units, and said capacity control means comprises arefrigerant flow controlling unit associated with said firstrefrigeration unit for variably controlling the flow of refrigerant insaid first refrigeration unit to thereby control the cooling capacity ofsaid unit in response to operation of said third signal responsivemeans.
 11. A system as claimed in claim 10 and further including zoneset point adjusting circuitry for establishing an adjustable zone setpoint temperature, said zone set point adjusting circuitry comprising anadjustable impedance element for shifting the level of said airtemperature responsive signals relative to a reference voltage tothereby change the zone set point temperature, and wherein the levels ofsaid first through sixth output signals vary from said reference voltagelevel in one sense direction.
 12. A method of controlling airtemperature in an air conditioned zone comprising: a. providing firstand second refrigeration units; b. directing air across air cooling heatexchangers of said units and into said zone; c. producing an electricair temperature responsive signal which varies in magnitude relative toa reference level in accordance with fluctuations in sensed airtemperature, said air temperature responsive signal increasing inmagnitude relative to the reference level in response to increases insensed air temperature and decreasing in magnitude relative to thereference level in response to decreases in sensed air temperature; d.initiating operation of said first refrigeration unit in response toproduction of a first air temperature signal having a first magnituderelative to said reference level and terminating operation of said firstunit in response to production of a second air temperature signal havinga second magnitude less than the magnitude of said first signal; e.initiating operation of said second refrigeration unit in response to athird air temperature signal level having a greater magnitude than saidfirst air temperature signal level and terminating operation of saidsecond unit at a fourth air temperature signal level having a greatermagnitude than said second air temperature signal level and a lessermagnitude than said first air temperature signal level; and, f.modulating the cooling capacity of said first unit between full capacityand a lesser capacity in accordance with air temperature signals rangingbetween fifth and sixth levels, said range of modulating signals betweensaid fifth and sixth levels being of lesser magnitude than said firstair temperature signal level and greater magnitude than said Fourth airtemperature signal level whereby the capacity of said firstrefrigeration unit is fully modulatable when said first unit isoperating and when said first and second units are operating.